1. Field of the Invention
The present invention relates to a control system for an automatic transmission.
2. Related Art
Automatic transmissions are conventionally combined with torque converters, acting as fluid couplings, for transmitting the rotational output of the engine to the transmission. The conventional transmission is equipped with at least one planetary gear unit composed of a plurality of gear elements for effecting shifting in accordance with a shift pattern which is preset to correspond to the vehicle speed, the throttle opening and so on.
The conventional automatic transmission provides a (parking) P-range, a (reverse) R-range, a (neutral) N-range, a (drive) D-range, a (second) S-range, a (low) L-range and so on. If the range is switched from the N-range to the D-range responsive to movement of the shift lever, for example, the rotation of the engine in the idling state is transmitted through the torque converter to the transmission, to cause a creep phenomenon in which the vehicle will advance little by little without any depression of the accelerator pedal.
Therefore, when it is detected that a range such as the D-range, the S-range or the L-range (hereinafter each will be called a "forward range") for running the vehicle forward is selected, that the accelerator pedal is released, that the brake pedal is depressed and that the vehicle speed is substantially "0", the forward clutch which is applied in forward running, i.e., a first clutch, is brought into slipping engagement to establish a pseudo-neutral state (hereinafter referred to as the "neutral control state") so that the aforementioned creep phenomenon may be prevented.
While idling in the D-range, the oil pressure fed to the hydraulic servo of the first clutch is lowered to establish slipping engagement so that the difference in the R.P.M. between the input and output of the torque converter will become a predetermined value for preventing the creep phenomenon (as disclosed in U.S. Pat. No. 4,850,251).
For this purpose, a signal pressure is produced so that a regulating oil pressure is produced by a regulator valve in response to the signal pressure. Moreover, a change-over valve is provided to selectively feed the regulated oil pressure or the D-range pressure to the hydraulic servo of the first clutch so that either the ordinary shift state or the neutral control state may be selected.
In the neutral control state, it is possible, when the accelerator pedal is depressed, to again apply the first clutch, for example, to prevent the delay in the application due to stroke loss of the clutch piston, racing of the engine, and shock of engagement.
In the aforementioned control system for an automatic transmission of the prior art, however, the change-over valve selectively establishes communication of its various ports by reciprocation of its spool. As a result, during switching of the change-over valve, there is a brief interval in which neither the regulated oil pressure nor the D-range pressure is fed to the hydraulic servo of the first clutch, so that the oil pressure of the hydraulic servo temporarily drops. Such a temporary drop in oil pressure might cause the first clutch to abruptly release, thereby providing a release shock. Further, if the first clutch is abruptly applied, after the abrupt release, an engaging shock may also occur.